1. Field of the Invention
The present invention relates in general to a high pressure sodium lamp having an arc tube and sodium amalgam sealed in the arc tube, and in particular to a relationship between the arc tube structure and the amount of sodium amalgam sealed in the arc tube.
2. Description of the Background
In general, high pressure sodium lamps typically include a ceramic arc tube in which an amount of xenon gas and a sodium amalgam are sealed. The sodium in the sodium amalgam sealed in the arc tube gradually reacts with ceramic of the arc tube, and thus, some amount of sodium is lost during an operational life period of the lamp. To compensate for the amount of the lost sodium, an excess amount of sodium amalgam is sealed in the arc tube in advance. It is conventionally believed that the greater the amount of sodium amalgam sealed in the arc tube, the better.
FIG. 1 shows one example of a conventional high pressure sodium lamp. A soldered alumina plug 11 includes a central aperture 13 through which a thin-walled niobium tube 15 penetrates at a short distance. Niobium tube 15 is hermetically sealed through central aperture 13 by a sealing composition, e.g., glass solder, indicated by a thick line at 17. Niobium tube 15 acts as an exhaust tube and as an inlead. Plug 11 has its neck portion extending into an alumina arc tube 19, and the edge portion of tube 19 butts against the solder portion of plug 11. The contact portion between arc tube 19 and plug 11 is hermetically sealed by a sealing composition, e.g., glass solder, indicated at 21 and 23. A coiled electrode 25 is fixed on the top portion of niobium tube 15 located in arc tube 19. The inside of niobium tube 15 is in fluid communication with the inside of arc tube 19 through an aperture 27 formed at the side wall of niobium tube 15. The outer end portion 29 of niobium tube 15 is squeezed after exhausting air in arc tube 19 and niobium tube 15. At this time, an excess sodium amalgam 30 is provided in arc tube 19 and niobium tube 15. The sodium amalgam is accumulated at the inside of outer end portion 29 because it is at a low temperature.
In the above-described high pressure sodium lamp, since the outer end portion 29 of niobium tube 15 outwardly extends from arc tube 19, the temperature of the outer end portion 29 hardly rises. Furthermore, since undesirable trenches or unevenness occur in the inner surface of niobium tube 15 during the forming process, liquidized sodium amalgam tends to move into arc tube 19 along such trenches by a capillary action, and therefore, the lamp characteristics of the sodium lamp are adversely affected.
FIG. 2 shows another example of a conventional high pressure sodium lamp. In this case, a high pressure sodium lamp includes a monolithic tube 31 composed of an alumina arc tube portion 33 and an alumina plug portion 35 integrally formed with the arc tube portion 33. Alumina plug portion 35 of monolithic tube 31 has a central aperture 37 through which a niobium tube 39 penetrates at a short distance. Niobium tube 39 and alumina plug portion 35 are hermetically sealed by a sealing composition, e.g., glass solder, indicated by a thick line at 41. An electrode 43 is fixed on the penetrating end portion of niobium tube 39.
In the high pressure sodium lamp shown in FIG. 2, since no aperture permitting fluid communication between the inside of niobium tube 39 and the inside of arc tube 31 is provided in the surface of niobium tube 39, sodium amalgam 45 is liquidized and stays in a ring-shaped state along an inner end corner 45 of monolithic tube 31, the temperature of which is lower than that of remaining portions thereof. In this case, since the low temperature portion is part of monolithic tube 31, the temperature thereof rises easily. Furthermore, since alumina monolithic tube 31 seldom has trenches or unevenness at the inner surface thereof, undesirable movement of sodium amalgam does not occur. Characteristics of the lamp are rarely changed.
However, in this type of lamp shown in FIG. 2, the end portion of monolithic tube 31 was intensely blackened after 3,000 hours elapsed in an operational life period when the diameter of monolithic tube 31 was reduced or the amount of sodium amalgam sealed in monolithic tube 31 was increased. Thus, the lamp voltage of the lamps having monolithic tube 31 greatly increased, and some of the lamps resulted in cycling. This is because, as determined according to the present invention, a part of sodium amalgam 45 is in contact with glass solder 41, and thereby, sodium of sodium amalgam 45 reacts to a component of glass solder 41.
Japan Laid-open patent application (KOKAI) 58-140963 discloses a high pressure sodium lamp assembly shown in FIG. 3. A monolithic arc tube 51 made of ceramic includes a hollow body portion 53 and a plug portion 55 having a central aperture 57. A ring-shaped inner wall 59 extends from the edge of central aperture 57 toward the inside of arc tube 61. A niobium tube 61 penetrates through central aperture 57, and is hermetically sealed by a sealing composition 62, e.g., glass solder. An electrode 63 is fixed on the top portion of niobium tube 61. In the above-described sodium lamp, ring-shaped inner wall 59 prevents sodium amalgam from being in contact with electrode 63. Ring-shaped inner wall 59 also prevents sodium amalgam from being in contact with sealing composition 62. However, since the constitution of plug portion 55 having ring-shaped inner wall 59 is complicated, it is technically difficult to mass produce such a monolithic tube.